1. Field of the Invention
The present invention relates to a communication system, particularly a mobile communication system, wherein a control link between a switchboard station and a radio base station can be established by control signals by using ATM (Asynchronous Transfer Mode) cells.
2. Description of the Prior Art
A mobile communication system includes switchboard stations which are connected with public network and with a plurality of radio base stations which are connected with a switchboard station. In order to establish the control link between a switchboard station and a radio base station, ATM information transmission has been utilized.
In ATM information transmission, fixed length packet (ATM cell) is used. A 24 bit storage region for VPI(Virtual Path Identifier)/VCI (Virtual Channel Identifier) is given at the heading field(header portion), to store the address of the transmitter.
Particularly, in case of establishing a control link between a switchboard station and each radio base station by utilizing ATM information transmission, the switchboard station writes the specific VPI/VCI values which are assigned to the radio base station with whom the switchboard station want to establish control link in the VPI/VCI region of an ATM cell which is to be transmitted.
For example, the radio base station shown in FIG. 4 has VPI/VCI comparison unit 41 and inter- station control signal processing unit 42. After receiving an ATM cell, the radio base station compares the VPI/VCI values which are included in the received ATM cell with the VPI/VCI values stored in VPI/VCI comparison unit 41.
When they are found to be identical, the radio base station recognizes that the ATM cell is directed to itself and then outputs the received ATM cell to the inter-station control signal processing unit 42.
Thus, in the ATM communication, information transmission can not be executed, unless the receiving side (the radio base station) knows the specific VPI/VCI values.
However, the load of the radio base station for assigning and confirming the specific VPI/VCI values is heavy.
Therefore, hitherto, in order to reduce the load of the radio base station, special VPI/VCI values called representative VPI/VCI values have been assigned beforehand to the radio base station which prepares to open.
The representative VPI/VCI values are given to all the newly established radio base stations and are different from the specific VPI/VCI values peculiar to each radio base station.
The switchboard station transmits, to a newly established radio base station, an ATM cell in which a representative VPI/VCI values are written in the VPI/VCI storage region.
The radio base station compares the representative VPI/VCI values with the VPI/VCI values memorized by itself. Here, because the VPI/VCI values memorized by the radio base station are, as explained above, the representative VPI/VCI values, the radio base station can receive an ATM cell transmitted by the switchboard station. Then, the radio base station requests a formal (proper) VPI/VCI values. Thus, a temporal link between the switchboard station and the radio base station is established. Hereafter, the switchboard station notices the formal VPI/VCI values to the radio base station. The radio base station, after receiving the formal VPI/VCI values, memorizes them to communicate by using the formal VPI/VCI values, hereafter.
However, the prior art as explained above has a disadvantage that it spares much time to establish a link between the switchboard station and the radio base station. Further, the formal link can not be established, unless a temporal link is established by using a representative VPI/VCI.
Furthermore, the prior art as explained above has another disadvantage that there is a possibility of failure in the link establishment, because it is necessary to execute complex control procedures, such as an alteration of the designation of the switch address (path setting) in an ATM switch that switches ATM cells in the switchboard station.
Still, the prior art as explained above has a disadvantage that the resource of VPI/VCI values is reduced, because special representative VPI/VCI values must be preserved.
Therefore, an object of the present invention is to provide a communication system which can establish a control link between the radio base station and the switchboard station, without assigning beforehand a representative or proper VPI and VCI values to the radio base station which is to be newly established.
Another object of the present invention is to provide a communication system that can shorten the time for establishing a control link between the switchboard station and the radio base station.
In accordance with the present invention, there is provided an ATM communication system, which comprises a first station for transmitting ATM cell, and a second station connected with the first station for receiving said ATM cell.
The second station comprises a comparison means for comparing the VPI and VCI which are written in said ATM cell with proper VPI and VCI of said second station, a signal processing means for receiving said ATM cell, when said ATM cell is identical with said proper VPI and VCI, and a mask set up means for invalidating the comparison result in said comparison means.
In this system, ATM cell can be received, even when proper VPI/VCI are not yet assigned to the second station.
There is also provided a method for establishing a link between the radio base station and the switchboard station.
This method comprises the steps of receiving all the ATM cells transmitted by the switchboard station, regardless of the VPI and VCI written in the ATM cells, until the radio base station receives the ATM cell which includes a link establishment request signal, comparing the VPI and VCI which are written in the ATM cell with proper VPI and VCI for the radio base station, and establishing control link by transmitting ATM cell from the switchboard station to the radio base station, when the VPI and VCI written in received ATM cell is found to be identical with said proper VPI and VCI.
After all, the switchboard station transmits a cell including the link establishment request signal to a newly established radio base station. The radio base station can receive the cell, regardless of the VPI/VCI values written in the cell header, because a mask set up unit in the radio base station sets mask approval, beforehand.
Further, the radio base station, after receiving the cell including the link establishment request signal, stores the VPI and VCI values included in that cell in a down cell header information memory, and transmits a response signal cell in which this VPI and VCI values is written in the cell header.
According to the present invention, the time required to establish a link between a switchboard station and a radio base station can be greatly reduced. The reason lies in that the mask set up unit which invalidates the comparison result is provided in the VPI/VCI comparison unit of the radio base station in order to enable the radio base station to receive ATM cell from the switchboard station without identifying the VPI/VCI values assigned to itself. In other words, the VPI/VCI values which is to be assigned to a radio base station are inserted in the above mentioned ATM cell.
Further, according to the present invention, it can never occur that the link is not established. The reason lies in that the switchboard station decides VPI and VCI values which are to be assigned to a radio base stations, and sets up the communication path by using this VPI/VCI values.
Furthermore, according to the present invention, the VPI/VCI resource is never wasted. The reason lies in that the mask set up unit which invalidates the comparison result is provided in the VPI/VCI comparison unit of radio base station in order to enable the radio base station to receive ATM cell from the switchboard station without identifying the VPI/VCI values assigned to itself. In other words, the necessity of special representative VPI and VCI is eliminated.